Thin film transistor array gate electrode for liquid crystal display device

ABSTRACT

The present invention discloses a TFT array substrate that is fabricated using a four-mask process and a method of manufacturing that TFT array substrate. The gate line and gate electrode of the array substrate is surrounded by the metallic oxide after finishing a first mask process using thermal treatment. As a result, the gate line and gate electrode are not eroded and damaged by the etchant and stripper during a fourth mask process. Further, buffering layer can optionally be formed between the substrate and the gate line and gate electrode. Thus, silicon ions and oxygen ions included in the substrate are not diffused into the gate line and electrode. Accordingly, the line defect such as a line open of the gate line and gate electrode is prevented, thereby preventing inferior goods while increasing the manufacturing yield.

RELATED APPLICATION

This application is a Rule 1.53(b) Divisional Application of applicationSer. No. 09/972,963 filed Oct. 10, 2001 now U.S. Pat. No. 6,765,270, andalso claims the benefit of Korean Patent Application No. 2000-59429,filed on Oct. 10, 2000, which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display (LCD) device,and more particularly, to a thin film transistor (TFT) array substrateand a method of manufacturing the same.

2. Discussion of the Related Art

A liquid crystal display device uses the optical anisotropy andpolarization properties of liquid crystal molecules to produce an image.Liquid crystal molecules have a definite orientational alignment as aresult of their long, thin shapes. That orientational alignment can becontrolled by an applied electric field. In other words, as an appliedelectric field changes, so does the alignment of the liquid crystalmolecules. Due to the optical anisotropy, the refraction of incidentlight depends on the orientational alignment of the liquid crystalmolecules. Thus, by properly controlling an applied electric field adesired light image can be produced.

A liquid crystal is classified into a positive liquid crystal and anegative liquid crystal, in view of an electrical property. The positiveliquid crystal has a positive dielectric anisotropy such that long axesof liquid crystal molecules are aligned parallel with an electric field.Whereas, the negative liquid crystal has a negative dielectricanisotropy such that long axes of liquid crystal molecules are alignedperpendicular to an electric field.

While various types of liquid crystal display devices are known, activematrix LCDs having thin film transistors and pixel electrodes arrangedin a matrix are probably the most common. This is because such activematrix LCDs can produce high quality images at reasonable cost.

FIG. 1 shows the configuration of a typical TFT-LCD device. The TFT-LCDdevice 11 includes upper and lower substrates 5 and 22 with aninterposed liquid crystal 14. The upper and lower substrates 5 and 22are called a color filter substrate and an array substrate,respectively.

In the upper substrate 5, on a surface opposing the lower substrate 22,black matrix 6 and color filter layer 7 that includes a plurality of red(R), green (G), and blue (B) color filters are formed in shape of anarray matrix such that each color filter 7 is surrounded by the blackmatrix 6. Further on the upper substrate 5, a common electrode 18 isformed and covers the color filter layer 7 and the black matrix 6.

In the lower substrate 22, on a surface opposing the upper substrate 5,thin film transistors (TFTs) “T”, as switching devices, are formed inthe shape of an array matrix corresponding to the color filter layer 7,and a plurality of crossing gate and data lines 13 and 15 are positionedsuch that each TFT “T” is located near each crossover point of the gateand data lines 13 and 15. Further in the lower substrate 22, a pluralityof pixel electrodes 17 are formed on an area defined by the gate anddata lines 13 and 15. The area there defined is called a pixel region“P”. The pixel electrode 17 is usually formed from a transparentconductive material having good transmissivity, for example,indium-tin-oxide (ITO) or indium-zinc-oxide (IZO).

The pixel and common electrodes 17 and 18 generate electric fields thatcontrol the light passing through the liquid crystal cells. Bycontrolling the electric fields desired characters or images aredisplayed.

To complete the array substrate described above, a depositing technique,a photolithography technique, and an etching technique are repeatedseveral times. Namely, a typical TFT array substrate manufacturingprocess requires repeated steps of depositing and patterning variouslayers. The patterning steps involve photolithography masks. Eachphotolithography step is facilitated using one mask, and the number ofmasks used in the fabrication process is a critical factor indetermining the number of patterning steps. Thus, the production costdepends heavily on the number of masks used in the manufacturingprocess. Moreover, the margin of error caused by a plurality ofmanufacturing processes depends heavily on the number of masks, andthus, the ratio of inferior goods is also lowered if the number of themask is lowered.

Accordingly, the TFT array substrate, nowadays, tends to be fabricatedusing four mask processes instead of five mask processes. However, whenusing the four mask processes, a plurality of layers that are stackedupon each other are simultaneously etched. Also, the etching ratios ofthe different layers should be adjusted during the etching process. As aresult, some portions of the lines, such as the gate and data lines,become exposed and some portions of the electrodes, such as the source,drain and gate electrodes, are also exposed. Above all, since the gateline and the gate electrode are usually formed of a low-resistancematerial when fabricating the TFT array substrate using the four maskprocesses, the exposed low-resistance material is gradually eroded bythe etchant during the manufacturing processes.

Now, referring to the attached drawings, the erosion of the gate lineand gate electrode will be explained in detail hereinafter.

FIG. 2 is a schematic partial plan view showing pixels of the TFT arraysubstrate that is fabricated using four mask processes. As shown, theTFT array substrate includes a gate line 13 formed on a transparentsubstrate, a data line 15 perpendicularly crossing the gate line 13, aTFT “T” formed at regions near the crossover point of the gate and datalines 13 and 15, and a pixel electrode 17 connected to the TFT. A pixelregion where the pixel electrode 17 is positioned is defined by the gateand data electrodes 13 and 15.

Still referring to FIG. 2, the TFT “T” is comprised of a gate electrode31, a source electrode 33 and a drain electrode 35. The gate electrode31 is extended from the gate line 13 and the source electrode 33 isextended from the data line 13. Further, the drain electrode 35 isspaced apart from the source electrode 33 and a channel region “CH” isformed between the source and drain electrodes 33 and 35. The gateelectrode 31 and gate line 13 are formed using a first mask. The dataline 15 and source and drain electrodes 33 and 35 are formed using asecond mask. Also, the pixel electrode 17 is formed using a fourth mask.In the case of forming the TFT array substrate using the four maskprocesses, an active layer 37 is not formed independently using anotherpatterning process. Namely, the active layer 37 is simultaneously formedwhen a protection layer 41 is patterned using a third mask, and thus,the active layer are located along and under the data line 15, sourceelectrode 33 and drain electrode 35.

However, during the third mask process that patterns the protectionlayer 41, portions “B” and “C” of the gate electrode 31 are exposed.Thereafter, these exposed portions “B” and “C” of the gate electrode 31are eroded by the stripper that removes the photo resist and by theetchant that removes a metallic layer during the fourth mask process.

For further explanation, a manufacturing process of the TFT arraysubstrate is explained referring to FIG. 3A to 3D.

FIGS. 3A to 3D are plan views and FIGS. 4–11 are correspondingcross-sectional views that relate to lines III—III and IV—IV of relatedart FIG. 2, and illustrate a process for manufacturing a related art TFTarray substrate for use in the liquid crystal display device.

FIGS. 3A, 4 and 5 show a first mask process. As shown, a first metallayer, for example copper (Cu), is deposited on a substrate 22, and thenpatterned so as to form the gate line 13 and gate electrode 31 using afirst mask. After that, a gate insulation layer 32, an amorphous siliconlayer 34, an impurity-included amorphous silicon layer 36, and a secondmetal layer 38 are deposited in series on a surface of the substrate 22having the gate line 13 and gate electrode 31.

FIGS. 3B, 6 and 7 show a second mask process. As shown, the second metallayer 38 of FIGS. 4 and 5 is patterned so as to form the data line 15,the source electrode 33 and the drain electrode 35. Again, the data line15 is perpendicular to the gate line 13 and the source electrode 33 isextended from the data line 15 over the pixel region “P” of FIG. 2.Also, the drain electrode 35 formed in the pixel region is spaced apartfrom the source electrode 33.

Next, the impurity-included amorphous silicon layer 36 of FIGS. 4 and 5is patterned using the patterned second metal layer as masks. Thus, anohmic contact layer 39 is formed under the patterned second metal layersuch as the data line 15 and the source and drain electrodes 33 and 35.Moreover, a portion of the amorphous silicon layer 34 between the sourceand drain electrodes 33 and 35 is exposed so as to form the channelregion “CH”.

FIGS. 3C, 8 and 9 show a third mask process. The protection layer 41 isdeposited on the amorphous silicon layer 34 of FIGS. 6 and 7 and on thepatterned second metal layer. After that, a drain contact hole 43 isformed by patterning the protection layer 41. At this time, theprotection layer 41 is mostly removed except portions that protect thedata line 15, channel region “CH” and source and drain electrodes 33 and35. Also, the amorphous silicon layer 34 of FIGS. 6 and 7 and the gateinsulation layer 32 are simultaneously removed except the portions underthe channel region “CH” and under the patterned second metal layer(i.e., the data line 15 and the source and drain electrodes 33 and 35).Thus, under the patterned protection layer 41, the active layer 37 isformed. Further, in this structure of the TFT, since this active layer37 does not cover the whole gate electrode 31 in order to form thechannel region “CH”, the protection layer 41 and the active layer 37exist between the source electrode 33 and the drain electrode 35.

As a result, since the channel region “CH” is formed between the sourceand drain electrodes 33 and 35, the portions “B” and “C” of the gateelectrode 31 are exposed after performing this third mask process.Moreover, the gate line 13 is also exposed.

FIGS. 3D, 10 and 11 show a fourth mask process. A transparent conductivematerial, such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO), isdeposited on the surfaces of the above-mentioned intermediates.Thereafter, the transparent conductive material is patterned to form thepixel electrode 17 that is electrically connected with the drainelectrode 35 through the drain contact hole 43.

As described above, after performing the third mask process, the exposedgate electrode portions “B” and “C” and the gate line 13 are eroded anddamaged during the fourth mask process. Namely, when forming the pixelelectrode 17, the exposed gate line 13 and gate electrode portions “B”and “C” are eroded and damaged by the etchant that etches thetransparent conductive material. Subsequently, when removing the photoresist that is formed for the pixel electrode 17, the exposed gate line13 and gate electrode portions “B” and “C” are secondly eroded anddamaged by the stripper.

Further, if the gate line and the gate electrode are made of copper(Cu), the copper ions are diffused into the liquid crystal layer afterthe liquid crystal panel is complete. Thus, the liquid crystal displaydevice malfunctions due to the diffused copper ions.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a method ofmanufacturing a thin film transistor that substantially obviates one ormore of the problems due to limitations and disadvantages of the relatedart.

An object of the present invention is to provide a method ofmanufacturing a TFT array substrate (as well as a TFT array substrateitself) that prevents the erosion and damage of the gate electrode andgate line.

Another object of the present invention is to provide a method ofmanufacturing a TFT array substrate (as well as a TFT array substrateitself) that prevents the diffusion of the copper ions from the gateline and gate electrode.

Another object of the present invention is to provide a method ofmanufacturing a TFT array substrate (as well as a TFT array substrateitself) that prevents the gate line and gate electrode from diffusion ofsilicon ions and oxygen ions.

In order to achieve the above object, the preferred embodiment of thepresent invention provides a TFT array substrate for use in a liquidcrystal display device, including: a gate line arranged in a transversedirection over a substrate; a metallic oxide layer surrounding the gateline; a data line arranged in a longitudinal direction perpendicular tothe gate line over the substrate; a thin film transistor formed near thecrossing of the gate and data lines, the thin film transistorcomprising: a gate electrode over the substrate, the gate electrodebeing extended from the gate line and surrounded by the metallic oxide;a gate insulation layer on the metallic oxide surrounding the gateelectrode; an active layer and an ohmic contact layer formed on the gateinsulation layer; a source electrode formed on the ohmic contact layerover the gate electrode and extended from the data line; and a drainelectrode formed on the ohmic contact layer over the gate electrode andspaced apart from the source electrode; a protection layer formed overthe thin film transistor, the protection layer having a drain contacthole that exposes a portion of the drain electrode; and a pixelelectrode formed in a pixel region that is defined by the gate and datalines, the pixel electrode contacting the drain electrode through thedrain contact hole.

The metallic oxide is one of tantalum oxide (TaO_(X)), chrome oxide(CrO_(X)), titanium oxide (TiO_(X)) and tungsten oxide (WO_(X)).

A TFT array substrate for use in a liquid crystal display device furtherincludes a buffering layer between the substrate and the gate line andgate electrode. The metallic oxide is made of one of tantalum oxide(TaOX) and titanium oxide (TiOX) that are respectively made fromtantalum (Ta) and titanium (Ti) preferably using an oxidation reactionat a temperature of greater than 400° C. The buffering layer is one oftantalum nitride (TaN) and titanium nitride (TiN). Moreover, thebuffering layer alternatively can be one of silicon nitride (SiN_(X))and silicon oxide (SiO₂).

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a method ofmanufacturing a TFT arrays substrate for use in a liquid crystal displaydevice, comprising: forming a first metal layer over a substrate;forming a second metal layer on the first metal layer; patterning thefirst and second metal layers so as to form a gate line and a gateelectrode; thermally-treating the substrate having the patterned firstand second metal layers so as to diffuse material from the patternedfirst metal layer over the patterned first metal layer and then to forma metallic oxide layer surrounding the second metal layer by oxidizingthe diffused material of the first metal layer; forming a gateinsulation layer on the substrate, the gate line and the metallic oxidelayer; forming an amorphous silicon layer on the gate insulation layer;forming an impurity-doped amorphous silicon layer on the amorphoussilicon layer; forming a third metal layer on the impurity-includedamorphous silicon layer; patterning the third metal layer so as to forma data line, a source electrode and a drain electrode; patterning theimpurity-doped amorphous silicon layer using the patterned third metallayer as masks so as to form an ohmic contact layer and a channel regionin the amorphous silicon layer between the source and drain electrodes;forming a protection layer on the amorphous silicon layer and on thepatterned third metal layer; patterning the protection layer, theamorphous silicon layer and the gate insulation layer except portionsthat correspond to the patterned third metal layer and channel region;depositing a transparent conductive material in a pixel region that isdefined by the gate and data lines; and patterning the transparentconductive material so as to form a pixel electrode that contacts thedrain electrode.

The first metal layer is one of tantalum (Ta), chrome (Cr), titanium(Ti) and tungsten (W). These materials become the metallic oxide layer,i.e., the metallic oxide is one of tantalum oxide (TaOX), chrome oxide(CrOX), titanium oxide (TiOX) and tungsten oxide (WOX) after finishingthe thermal treatment. The second metal layer is copper (Cu). The thirdmetal layer is one of chrome (Cr), tantalum (Ta), titanium (Ti),tungsten (W) and molybdenum (Mo).

A method of manufacturing a TFT arrays substrate for use in a liquidcrystal display device further includes forming a buffering layer on thesubstrate before forming the first metal layer. The thermal treatment ispreferably performed at a temperature of greater than 400° C. Thebuffering layer can be one of tantalum nitride (TaN), titanium nitride(TiN) silicon nitride (SiN_(X)) and silicon oxide (SiO₂).

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 shows the configuration of a typical related art TFT-LCD device;

FIG. 2 is a schematic partial plan view showing pixels of the TFT arraysubstrate that is fabricated using four mask processes according to therelated art;

FIGS. 3A to 3D are plan views and FIGS. 4–11 are correspondingcross-sectional views that illustrate a process for manufacturing a TFTarray substrate for use in the liquid crystal display device accordingto the related art;

FIGS. 12A to 12D are plan views that illustrate a process according to afirst embodiment of the invention for manufacturing a TFT arraysubstrate for use in the liquid crystal display device;

FIGS. 13A to 13C and 14A to 14C are cross-sectional views cut alonglines XIII—XIII and XIV—XIV of FIG. 12A, respectively;

FIGS. 15 and 16 are cross-sectional views cut along lines XV—XV andXVI—XVI of FIG. 12B, respectively;

FIGS. 17 and 18 are cross-sectional views cut along lines XVII—XVII andXVIII—XVIII of FIG. 12C, respectively;

FIGS. 19 and 20 are cross-sectional views cut along lines XIX—XIX andXX—XX of FIG. 12D, respectively; and

FIGS. 21A to 21C and 22A–22C are cross-sectional views that illustrate aprocess according to a second embodiment of the invention formanufacturing a TFT array substrate for use in the liquid crystaldisplay device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, example of which is illustrated in the accompanyingdrawings.

FIGS. 12A to 12D are plan views and FIGS. 13S–20 are cross-sectionalviews that relate to lines III—III and IV—IV of related art FIG. 2, andillustrate a process according to a first embodiment of the inventionfor manufacturing a TFT array substrate similar to related art FIG. 2for use in the liquid crystal display device. In this description, theplan views of the preferred embodiment are similar to the conventionalart described in FIGS. 2–3D.

FIGS. 12A, 13A–13C and 14A–14C show a first mask process, wherein afirst metal layer 111 is formed at first on a substrate 100 bydepositing a metallic material such as of tantalum (Ta), chrome (Cr),titanium (Ti), tungsten (W) and the like. These metallic materials caneasily diffuse at a temperature on and along the surface of a secondmetal layer that is formed in a later step. After that, the second metallayer 130, such as copper (Cu), is deposited on the first metal layer111. And then, the second metal layer 130 is patterned so as to form thegate line 113 and gate electrode 131 using a first mask. At this time,the portions of the first metal layer 111 corresponding to the gate line113 and gate electrode 131 are simultaneously patterned (see FIGS. 13Band 14B).

Next, the substrate 100 having the gate line 113 and the gate electrode131 is thermally-treated at a designated temperature (again, see FIGS.13B and 14B). This thermal treatment diffuses the material of thepatterned first metal layer along the surface of the patterned secondmetal layer (for example, copper (Cu)) so that material of the firstmetal layer exists on the surface of the patterned second metal layer.At this time, the patterned first metal layer reacts with oxygen (O) andbecomes a metallic oxide 112 such as tantalum oxide (TaO_(X)), chromeoxide (CrO_(X)), titanium oxide (TiO_(X)), tungsten oxide (WO_(X)) orthe like. In other words, the metallic oxide layer 112 caused by theoxidation reaction is formed around the gate line 113 and gate electrode131 during the thermal treatment process (see the result in FIGS. 13Cand 14C).

After that, a gate insulation layer 132 is formed on the substrate 100and on the metallic oxide layer 112 that surrounds the gate line 113 andgate electrode 131 (see FIGS. 13C and 14C). The gate insulation layer132 is an insulation material, e.g., an organic material, such asbenzocyclobutene (BCB) or a material related to or containingacryl-based resin, or an inorganic material, such as silicon oxide(SiO_(X)) or silicon nitride (SiN_(X)). Further, an amorphous siliconlayer 134, an impurity-included amorphous silicon layer 136, and a thirdmetal layer 138 are deposited in series over the gate insulation layer132. The third metal layer 138 is a metallic material such as chromium(Cr), tantalum (Ta), titanium (Ti), tungsten (W), molybdenum (Mo) andthe like.

FIGS. 12B, 15 and 16 show a second mask process. As shown, the thirdmetal layer 138 of FIGS. 13C and 14C is patterned so as to form the dataline 115, the source electrode 133 and the drain electrode 135. The dataline 115 is perpendicular to the gate line 113 and the source electrode133 is extended from the data line 115 over the pixel region “P” of FIG.2. Also, the drain electrode 135 formed in the pixel region is spacedapart from the source electrode 133.

Next, the impurity-included amorphous silicon layer 136 of FIGS. 13C and14C is patterned using the patterned third metal layer portions 115, 133and 135 as masks. Thus, an ohmic contact layer 139 is formed under thepatterned third metal layer (i.e., the data line 115 and the source anddrain electrodes 133 and 135). Moreover, a portion of the amorphoussilicon layer 134 between the source and drain electrodes 133 and 135 isalso exposed so as to form the channel region

FIGS. 12C, 17 and 18 show a third mask process. The protection layer 141is deposited on the amorphous silicon layer 134 of FIGS. 15 and 16 andon the patterned portions 115, 133 and 135 of the third metal layer. Thepassivation layer 141 is an insulation material, e.g., an organicmaterial, such as benzocyclobutene (BCB) or a material relating to orcontaining acryl-based resin, or an inorganic material, such as siliconoxide (SiO_(X)) or silicon nitride (SiN_(X)). After that, a draincontact hole 143 is formed by patterning the protection layer 141. Atthis time, the protection layer 141 is mostly removed except portionsthat protect the data line 115, channel region “CH” and source and drainelectrodes 133 and 135. Also, the amorphous silicon layer 134 of FIGS.15 and 16 and the gate insulation layer 132 are simultaneously removedexcept the portions corresponding to the channel region “CH” and theportions under the patterned third metal layer (i.e., the data line 115and the source and drain electrodes 133 and 135). Thus, under thepatterned protection layer 141, the active layer 137 is formed.

FIGS. 12D, 19 and 20 show a fourth mask process. A transparentconductive material, such as indium-tin-oxide (ITO) or indium-zinc-oxide(IZO), is deposited on the surfaces of the above-mentionedintermediates. Thereafter, the transparent conductive material ispatterned to form the pixel electrode 117 that is electrically connectedwith the drain electrode 135 through the drain contact hole 143. Thepixel electrode 117 is located in the pixel region “P” of FIG. 2.

According to a principle of the present invention, although the portions“B” and “C” of the gate electrode 131 and the gate line 113 are exposedduring the fourth mask process, they are not eroded and damaged by theetchant and stripper. That is because the gate line 113 and the gateelectrode 131 are protected by the metallic oxide layer 112 thatsurrounds the patterned second metal layer. Further, although the gateline 113 and the gate electrode 131 are made of copper (Cu), the copperions are not diffused into the liquid crystal layer after the liquidcrystal panel is complete due to the metallic oxide layer 112. Thus, themalfunction caused by the diffused copper ions into the liquid crystallayer does not occur any more in the liquid crystal display device.

Now, reference will be made in detail to a second preferred embodimentof the present invention, example of which is illustrated in theaccompanying drawings.

In the first embodiment described above, if the first metal layer istantalum (Ta) or titanium (Ti), some problems occur in the thermaltreatment process. The thermal-treating temperature is made greater than400° C. in order to diffuse tantalum (Ta) or titanium (Ti) on thesurface of the second metal layer (for example, copper (Cu)). Butsilicon ions and oxygen ions included in the substrate also get diffusedinto the patterned second metal layer. As a result, carrier mobility ofthe gate line and gate electrode is lowered. According to the principleof the second embodiment, a simple structure is adopted in order toprevent the decrease of the carrier mobility.

FIGS. 21A–21C and 22A–22C are cross-sectional views that relate to linesIII—III and IV—IV of FIG. 2, respectively, and illustrate a processaccording to the second embodiment of the invention for manufacturing aTFT array substrate similar to related art FIG. 2 for use in the liquidcrystal display device.

Referring to FIG. 21A and 22A, a buffering layer 110 is formed on thesubstrate 100 by depositing, e.g., tantalum nitride (TaN) or titaniumnitride (TiN). Alternatively, the buffering layer 110 can be made, e.g.,of silicon oxide (SiO₂) or silicon nitride (SiN_(X)). After that, afirst metal layer 111 and a second metal layer 130 are deposited inseries over the buffering layer 110 just as described in FIGS. 12A,13A–13C and 14A–14C.

Next, referring to FIGS. 21B and 22B, the buffering layer 110, the firstmetal layer 111 and the second metal layer 130 are patterned so as toform the gate line 113 and gate electrode 131. After that, the substrate100 having the gate line 113 and the gate electrode 131 isthermally-treated at a temperature of greater than 400° C.

Now, referring to FIGS. 21C and 22C, the above-mentioned thermaltreatment diffuses the first metal layer 111 of FIGS. 21B and 22B alongthe surface of the patterned second metal layer (i.e., the gate line 113and the gate electrode 131) so that material of the first metal layerexists on the surface of the patterned second metal layer. At this time,the first metal layer reacts with oxygen (O) and becomes a metallicoxide layer 112 such as tantalum oxide (TaO_(X)) or titanium oxide(TiO_(X)). In other words, the metallic oxide layer 112 caused by theoxidation reaction is formed around the gate line 113 and gate electrode131 after thermal treatment.

Here, the next processes of the second embodiment are omitted becausethey are the same as the processes depicted in FIGS. 12B–12D and 15–20.According to the second embodiment, the silicon ions and the oxygen ionsare prevented from being diffused from the substrate into the gate lineand into the gate electrode due to the buffering layer. In other words,the buffering layer protects the gate line and gate electrode againstdiffusion of the silicon ions and oxygen ions. Thus, the gate line andgate electrode are not deteriorated by these ions, and the carriermobility is not reduced.

Further, the principles of the present invention can be used in thein-plane switching mode liquid crystal display (IPS-LCD) device.

Accordingly, as described above, since the TFT array substrate for usein the liquid crystal display device is fabricated using the four-maskprocess without any erosion and damage in the gate line and gateelectrode, the line defect such as a line open or break of the gate lineand gate electrode is prevented. And, copper ions do not diffuse intothe liquid crystal layer after the liquid crystal display is complete.Thus, malfunction does not occur in the liquid crystal display device,thereby preventing inferior goods while increasing the manufacturingyield.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the method of manufacturing athin film transistor of the present invention without departing from thespirit or scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

1. A method of forming a TFT array substrate for use in a liquid crystaldisplay device, comprising: forming a first metal layer over asubstrate; forming a second metal layer on the first metal layer;patterning the first and second metal layers so as to form a gate lineand a gate electrode; thermally-treating the substrate having thepatterned first and second metal layers so as to diffuse material fromthe patterned first metal layer over the patterned second metal layerand then to form a metallic oxide layer surrounding the second metallayer by oxidizing the diffused material of the first metal layer;forming a gate insulation layer on the substrate, the gate line and themetallic oxide layer; forming an amorphous silicon layer on the gateinsulation layer; forming an impurity-doped amorphous silicon layer onthe amorphous silicon layer; forming a third metal layer on theimpurity-doped amorphous silicon layer; patterning the third metal layerso as to form a data line, a source electrode and a drain electrode;patterning the impurity-included amorphous silicon layer using thepatterned third metal layer as masks so as to form an ohmic contactlayer and a channel region in the amorphous silicon layer between thesource and drain electrodes; forming a protection layer on the amorphoussilicon layer and on the patterned third metal layer; patterning theprotection layer, the amorphous silicon layer and the gate insulationlayer except portions that correspond to the patterned third metal layerand channel region; depositing a transparent conductive material in apixel region that is defined by the gate and data lines; and patterningthe transparent conductive material so as to form a pixel electrode thatcontacts the drain electrode.
 2. The method according to claim 1,wherein the first metal layer is one of tantalum (Ta), chromium (Cr),titanium (Ti) or tungsten (W).
 3. The method according to claim 2,wherein the metallic oxide layer is one of tantalum oxide (TaO_(X)),chromium oxide (CrO_(X)), titanium oxide (TiO_(X)) or tungsten oxide(WO_(X)).
 4. The method according to claim 1, wherein the third metallayer is one of chromium (Cr), tantalum (Ta), titanium (Ti), tungsten(W) or molybdenum (Mo).
 5. The method according to claim 1, furthercomprising: forming a buffering layer on the substrate before formingthe first metal layer.
 6. The method according to claim 5, wherein thebuffering layer is one of silicon nitride (SiN_(X)) or silicon oxide(SiO₂).
 7. The method according to claim 1, wherein thermal treatment ofthe substrate is performed at a temperature of greater than 400° C.
 8. Amethod of forming a TFT array substrate for use in a liquid crystaldisplay device, comprising: forming a first metal layer over asubstrate; forming a second metal layer of copper (Cu) on the firstmetal layer; patterning the first and second metal layers so as to forma gate line and a gate electrode; thermally-treating the substratehaving the patterned first and second metal layers so as to diffusematerial from the patterned first metal layer over the patterned secondmetal layer and then to form a metallic oxide layer surrounding thesecond metal layer by oxidizing the diffused material of the first metallayer; forming a gate insulation layer on the substrate, the gate lineand the metallic oxide layer; forming an amorphous silicon layer on thegate insulation layer; forming an impurity-doped amorphous silicon layeron the amorphous silicon layer; forming a third metal layer on theimpurity-doped amorphous silicon layer; patterning the third metal layerso as to form a data line, a source electrode and a drain electrode;patterning the impurity-included amorphous silicon layer using thepatterned third metal layer as masks so as to form an ohmic contactlayer and a channel region in the amorphous silicon layer between thesource and drain electrodes; forming a protection layer on the amorphoussilicon layer and on the patterned third metal layer; patterning theprotection layer, the amorphous silicon layer and the gate insulationlayer except portions that correspond to the patterned third metal layerand channel region; depositing a transparent conductive material in apixel region that is defined by the gate and data lines; and patterningthe transparent conductive material so as to form a pixel electrode thatcontacts the drain electrode.
 9. A method of forming a TFT arraysubstrate for use in a liquid crystal display device, comprising:forming a buffering layer on a substrate, the buffering layer being oneof tantalum nitride (TaN) or titanium nitride (TiN); forming a firstmetal layer over the buffering layer; forming a second metal layer onthe first metal layer; patterning the first and second metal layers soas to form a gate line and a gate electrode; thermally-treating thesubstrate having the patterned first and second metal layers so as todiffuse material from the patterned first metal layer over the patternedsecond metal layer and then to form a metallic oxide layer surroundingthe second metal layer by oxidizing the diffused material of the firstmetal layer; forming a gate insulation layer on the substrate, the gateline and the metallic oxide layer; forming an amorphous silicon layer onthe gate insulation layer; forming an impurity-doped amorphous siliconlayer on the amorphous silicon layer; forming a third metal layer on theimpurity-doped amorphous silicon layer; patterning the third metal layerso as to form a data line, a source electrode and a drain electrode;patterning the impurity-included amorphous silicon layer using thepatterned third metal layer as masks so as to form an ohmic contactlayer and a channel region in the amorphous silicon layer between thesource and drain electrodes; forming a protection layer on the amorphoussilicon layer and on the patterned third metal layer; patterning theprotection layer, the amorphous silicon layer and the gate insulationlayer except portions that correspond to the patterned third metal layerand channel region; depositing a transparent conductive material in apixel region that is defined by the gate and data lines; and patterningthe transparent conductive material so as to form a pixel electrode thatcontacts the drain electrode.
 10. A method of forming an insulatedconductor structure for use in a TFT array substrate of a liquid crystaldisplay device, the method comprising: providing a substrate; forming afirst metal layer over a substrate; forming a second metal layer on thefirst metal layer; patterning the first and second metal layers so as toform a conductive line and a conductive electrode thus defining anintermediate structure; thermally treating said intermediate structureso as to diffuse material from the patterned first metal layer over thepatterned second metal layer and then to form a metallic oxide layersurrounding the patterned second metal layer; and forming an insulationlayer on the substrate, the conductive line and said metallic oxidelayer.
 11. The method according to claim 10, wherein the first metallayer is one of tantalum (Ta), chromium (Cr), titanium (Ti) or tungsten(W) and the metallic oxide is one of tantalum oxide (TaO_(X)), chromiumoxide (CrO_(X)), titanium oxide (TiO_(X)) or tungsten oxide (WO_(X)),respectively.
 12. The method according to claim 10, further comprising:forming a buffering layer on the substrate before forming the firstmetal layer.
 13. The method according to claim 12, wherein the bufferinglayer is one of tantalum nitride (TaN), titanium nitride (TiN), siliconnitride (SiN_(X)) or silicon oxide (SiO₂).
 14. A method of forming aninsulated conductor structure for use in a TFT array substrate of aliquid crystal display device, the method comprising: providing asubstrate; forming a first metal layer over a substrate; forming asecond metal layer of copper (Cu) on the first metal layer; patterningthe first and second metal layers so as to form a conductive line and aconductive electrode thus defining an intermediate structure; thermallytreating said intermediate structure so as to diffuse material from thepatterned first metal layer over the patterned second metal layer andthen to form a metallic oxide layer surrounding the patterned secondmetal layer; and forming an insulation layer on the substrate, theconductive line and said metallic oxide layer.